Method for washing immunoassay elements

Abstract

A method of dispensing wash solution to separate free from bound labels in a slide immunorate assay. The method comprises dispensing wash first as separate drops spaced to allow complete absorption prior to the next drop, followed by a time at which the rate of dispensed wash exceeds the rate of fluid uptake of the slide, to form a continuous stream. The first phase of this wash method provides a more complete separation of bound from free under the dispense tip than occurs if only a continuous stream is used throughout.

What is claimed is: 
     
       1. In a method of separating bound labeled immunoreactant used to detect a target analyte from free labeled immunoreactant used to detect said analyte in an immunoassay conducted on a test element containing dried reagents prior to contact with a sample liquid and having a determinable rate of liquid uptake, the method comprising the step of depositing a wash liquid onto a portion of said test element having a surface and containing both said bound and said free labeled immunoreactants so as to separate said bound from said free labeled immunoreactants; the improvement wherein the depositing step comprises the steps of a) initially depositing the wash liquid a drop at a time at a rate that is less than the rate of uptake of each drop by said portion of said test element, so that a minimal remaining meniscus of the wash liquid remains at the surface of said portion when the next of the wash liquid is applied, said drops having a volume no larger than about 3 μL; and   b), after a plural number of drops are applied as per step a), applying the wash liquid at a rate that exceeds the rate of uptake of liquid as it currently exists in the test element portion being contacted by the wash liquid in step b) only, more than said minimal remaining meniscus of the wash liquid forms and remains during the washing of said surface, to effect the separation of said bound from said free labeled immunoreactants, wherein the total wash volume is deposited within a time not exceeding about 60 seconds.       
     
     
       2. The method as defined in claim 1, wherein said drops of said step a) have a volume producing a contact diameter with said test element that is less than the contact diameter that would be produced if the depositing were in the form of a continuous stream. 
     
     
       3. The method as defined in claim 1, wherein said step b) comprises applying the wash liquid at a rate that causes said remaining meniscus size to increase over time until all the wash liquid is deposited. 
     
     
       4. The method as defined in claim 1, wherein said step b) comprises increasing the rate of application of the wash liquid above the rate used in said step a) so as to minimize the time required for said washing, until all the wash liquid is deposited. 
     
     
       5. The method as defined in claim 1, wherein said step b) applies the wash liquid at the same depositing rate used in said step a), but the uptake rate of said test element portion decreases over time between steps a) and b) to a value that is less than the rate of application of step b). 
     
     
       6. The method as defined in claim 1, wherein said drop volume is no larger than about 2 μL. 
     
     
       7. In a method of separating bound labeled immunoreactant used to detect a target analyte from free labeled immunoreactant used to detect said analyte in an immunoassay conducted on a test element containing reagents, the method comprising the step of depositing a wash liquid onto a portion of said test element containing both said bound and said free labeled immunoreactants; the improvement wherein the depositing step comprises the steps of a) repeatedly washing the portion of said test element at an approximate center of the contact of said deposited wash liquid by repeatedly passing a contact meniscus of the deposited wash liquid through said portion, and   b) thereafter expanding the contact meniscus out beyond said portion of the test element being washed, until all the wash liquid is deposited within a total depositing time no greater than about 60 seconds.     
     
     
       8. The method as defined in claim 7, wherein the volume of said repeated, deposited wash liquid producing said contact meniscus, is no larger than about 2 μL.